Barring a insurance company failure (historically unlikely if not unheard of), a SPIA is predictable - a bond/muni/corporate index is not. Clearly bond fund NAVs have far more downside risk than upside with interest rates at historic lows. I'm not advocating SPIAs or any other annuity, but for those who want some (floor) income (virtually) without risk, SPIAs are predictable where bond funds are not. Some people cannot stomach or afford much risk in retirement, some can.
And I can find few if any "experts" who are predicting we'll see the same kind of returns we've seen from 1926-2000 in the decades ahead with the underpinnings of our economy/global competitive landscape, though anything is possible.